Basic Communication Systems MODULE 1 Basic Communication Systems BDG(34)

Introduction Main purpose of an electronic communications system is to transfer information from one place to another. Electronic communications can be viewed as the transmission, reception and processing of information between two or more locations using electronic circuit/device. BDG(34)

Basic Components BDG(34)

Input Transducer: The message produced by a source must be converted by a transducer to a form suitable for the particular type of communication system. Example: In electrical communications, speech waves are converted by a microphone to voltage variation. Transmitter: The transmitter processes the input signal to produce a signal suits to the characteristics of the transmission channel. Signal processing for transmission almost always involves modulation and may also include coding. In addition to modulation, other functions performed by the transmitter are amplification, filtering and coupling the modulated signal to the channel. BDG(34)

Example: Loudspeaker, personal computer (PC), tape recorders. Channel: The channel can have different forms: The atmosphere (or free space), coaxial cable, fiber optic, waveguide, etc. The signal undergoes some amount of degradation from noise, interference and distortion Receiver: The receiver’s function is to extract the desired signal from the received signal at the channel output and to convert it to a form suitable for the output transducer. Other functions performed by the receiver: amplification (the received signal may be extremely weak), demodulation and filtering. Output Transducer: Converts the electric signal at its input into the form desired by the system user. Example: Loudspeaker, personal computer (PC), tape recorders. BDG(34)

Transmission Direction - Simplex: One direction only BDG(34)

Half Duplex Transmission Half duplex: Both directions but only one direction at a time BDG(34)

Full Duplex Transmission Full duplex: send and receive both directions at once BDG(34)

Baseband and Passband Data Transmissions – Data are transmitted between two DTEs in multiples of a fixed unit, typically of eight bits. Each character or byte is transmitted serially. Transmission modes: Characters; Octets (bytes). BDG(34)

For the receiving device to decode and interpret the bit string, it must be able to determine: the start of each bit cell – in order to sample the incoming signal in the middle of the bit cell and to determine what kind of bit it is: 0 or 1 bit (clock) Synchronization; the start and end of each element (character or byte) character (byte) synchronization; the start and end of each complete message block (called also frame) frame (block) synchronization BDG(34)

Asynchronous transmission There are two methods to accomplish these tasks, each one determined by whether the transmitter and receiver clocks are independent (asynchronous transmission) or synchronized (synchronous transmission). Asynchronous transmission Data to be transmitted are generated at random intervals (from the keyboard, for example). The receiver must be able to detect the beginning of each new character received each transmitted character or byte is encapsulated (framed) between two additional elements with different electrical representation: a start bit and a stop element. BDG(34)

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Synchronous transmission Having breaks between characters for the transmission of large blocks of data at higher bit rates is not efficient to transmit the code combinations that correspond to these characters one at a time without breaks. The receiver must have a clock synchronized with the transmitter clock. If it is not synchronized there will be errors in the recovered data. Hence need for timing information (in the transitions of the data signal, because the intervals between the data signal transitions are multiples of the bit intervals). BDG(34)

Synchronous Transmission – BDG(34)

Signalling rate At each instant the transmitted signal can be in one state from a finite set of states (ex. In the binary transmission, one of two states); The duration of the shortest state is named elementary interval (T) the signaling rate is defined as: Vs =1/T bauds. BDG(34)

Baseband – the band of frequencies occupied by the (data) signal before it modulates a carrier (or subcarrier) frequency in order to form the transmitted line or radio signal. The baseband, therefore, has a frequency content extending into direct current region. Baseband data can be transmitted hundreds or even thousands of meters (the transmission distance is limited by several factors) and this is commonly done on wire pair, which has a low-pass frequency transfer characteristic so that it permits data to be transmitted directly without need for frequency translating. BDG(34)

a good protection against noise; However, there is need for some line coding to ensure that the transmitted signal has the following features: no d.c. component and low frequency components, because the transmission equipment is connected to the transmission line by transformers and these transformers have large attenuation at small frequencies; small bandwidth, in order to use efficiently the useful bandwidth of the transmission line and to avoid the large attenuation of the line at high frequencies; a good protection against noise; presence of timing information (transitions), necessary to synchronize the receiver clock with the transmitter clock; no necessity for the receiving device to determine the absolute polarity of the data signal. BDG(34)

1. Reduction in the size of antenna. 2. Avoids mixing of signals. Need of Modulation - In the process of modulation, the baseband signal is ‘translated’ i.e. shifted from low frequency to high frequency. Advantages – 1. Reduction in the size of antenna. 2. Avoids mixing of signals. 3. Increase the range of communication 4. Multiplexing is possible. 5. Improves quality of reception. BDG(34)

Communication Channels Guided transmission media: communications signals guided along a solid medium Wireless media: communications signal broadcast over airwaves as a form of electromagnetic radiation BDG(34)

DIFFERENT TYPES OF COMMUNICATION MEDIA (CHANNELS) 1.Twisted Wire— Most prevalent form of communication wiring, used for most business telephone wiring, consists of strands of copper wire twisted in pairs. DIFFERENT TYPES OF COMMUNICATION MEDIA (CHANNELS) ADVANTAGES: LOW COST EASY TO WORK WITH DISADVANTAGES: RELATIVELY SLOW SUBJECT TO INTERFERENCE FROM OTHER ELECTRICAL SOURCES BDG(34)

2. Coaxial cable—used for cable television, consists of insulated copper wire. ADVANTAGES: FASTER THAN TWISTED WIRE LESS SUSCEPTIBLE TO ELECTROMAGNETIC INTERFERENCE DISADVANTAGES: RELATIVELY EXPENSIVE SOMEWHAT DIFFICULT TO WORK WITH BDG(34)

3. Fiber optic— Consists of clear glass fiber and transmits information in the form of light waves, instead of electric current. ADVANTAGES: CONSIDERABLY FAST SMALLER AND LIGHTER THAN COAXIAL CABLES DISADVANTAGES: EXPENSIVE HARD TO INSTALL AND DIFFICULT TO WORK WITH BDG(34)

4. WIRELESS COMMUNICATIONS Microwave— are Widely Used For High-volume, Long Distance, Point-to-point Communication. They Transmit High Frequency Radio Signals In The Atmosphere. Microwave Signals Follow A Straight Line Between Rely Stations 30 Miles Apart (Do Not Bend With Earth’s Curvature). This Limitation Makes Microwave Systems More Expensive. BDG(34)

B. Satellites — Transmits data using orbiting satellites. Satellites serve as rely stations for transmitting microwave signals over very long distances. Satellites are efficient way of transmitting large amount of data over a very long distance. BDG(34)

Guided Transmission Media Types BDG(34)

Wireless Technologies BDG(34)

Frequencies for communication 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz visible light VLF LF MF HF VHF UHF SHF EHF infrared UV optical transmission coax cable twisted pair VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wave length:  = c/f where; wavelength - , speed of light - c  3x108m/s, frequency f BDG(34)

Electromagnetic Frequency Spectrum BDG(34)

International Standards The spectrum is divided into bands, with each band having a different name and boundary. The radio frequency band (30Hz ~300GHz) is divided into narrower band as follow. BDG(34)

Wireless Communication Systems BDG(34)

Current Wireless Systems Cellular systems Wireless LANs Satellite Systems Paging Systems Bluetooth Ultrawideband Radios Zigbee Radios BDG(34)

Satellite Communication Systems BDG(34)

Communication Satellite Transponder BDG(34)

Optical fiber Communication Systems BDG(34)